1. Field of the Invention
The present invention relates to a method of controlling an air-fuel ratio of an engine.
2. Description of the Related Art
Known in the art is an internal combustion engine which finds a first angular velocity of the crankshaft during the time required for the crankshaft to rotate from 30.degree. to 60.degree. after top dead center of the compression stroke from this period, finds a second angular velocity of the crankshaft during the time required for the crankshaft to rotate from 90.degree. to 120.degree. after top dead center of the compression stroke from this period, finds the torque generated by a cylinder from the square of the first angular velocity and the square of the second angular velocity, and calculates the amount of fluctuation of the torque from the amount of fluctuation of the generated torque (see U.S. Pat. No. 4,691,286).
That is, when combustion is performed in a cylinder, the combustion pressure causes the angular velocity of the crankshaft to rise from a first angular velocity .omega.a to a second angular velocity .omega.b. At this time, if the moment of inertia of rotation of the engine is I, the combustion pressure causes the kinetic energy to rise from (1/2).multidot.I.omega.a.sup.2 to (1/2).multidot.I.omega.b.sup.2. Roughly speaking, the amount of rise of the kinetic energy (1/2).multidot.I.multidot.(.omega.b.sup.2 -.omega.a.sup.2) causes a torque to be generated, so the generated torque becomes proportional to (.omega.b.sup.2 -.omega.a.sup.2). Therefore, the generated torque is found from the difference between the square of the first angular velocity .omega.a and the square of the second angular velocity .omega.b and, therefore, in the above-mentioned internal combustion engine, the amount of fluctuation of the torque is calculated from the thus found generated torque.
However, if it is possible to calculate the amount of fluctuation of the torque in this way, it would also be possible to control the air-fuel ratio to the lean limit based on this amount of torque fluctuation. That is, when the air-fuel ratio is at the rich side of the lean limit, the amount of torque fluctuation is small, while when the air-fuel ratio is at the lean side of the lean limit, the amount of torque fluctuation becomes large, so by correcting the air-fuel ratio based on the amount of torque fluctuation so that the amount of torque fluctuation becomes within a predetermined range, it is possible to control the air-fuel ratio to the lean limit. This type of control of the air-fuel ratio becomes possible when the torque fluctuation calculated based on the first angular velocity ma and second angular velocity .omega.b expresses the torque fluctuation due to the fluctuation of the combustion pressure.
When the vehicle is driving over a rough road, however, the engine drive system experiences large amplitude torsional vibration and, as a result, the torque calculated based on the first angular velocity .omega.a and the second angular velocity .omega.b fluctuates considerably. The occurrence of the large torque fluctuation when torque fluctuation not due to the fluctuation of the combustion pressure occurs means that if the air-fuel ratio is corrected, the air-fuel ratio will end up deviating considerably from the lean limit. Therefore, it is necessary to prohibit correction of the air-fuel ratio when large torque fluctuations occur due to driving over a rough road.
The related art of the above internal combustion engines explained above, however, do not give any suggestions on the occurrence of torque fluctuations due to driving over a rough road and therefore application of the methods described in the above related art would result in the problem of the air-fuel ratio deviating from the target air-fuel ratio.